A scheme to encourage companies to open their loos to the public was launched today by Boris Johnson, the mayor of London, to help tackle the shortage of public toilets in the capital.

"Open London" stickers will be used to publicise firms willing to lend their toilets for free during opening hours without obliging individuals to make a purchase.

Johnson wants to alleviate the frustration of Londoners and tourists caught short in a city that has experienced more than double the average rate of public loo closures in Britain. There has been a 40% decline in the number public toilets in the capital since 1999.

The new scheme is part of the Tory mayor's manifesto commitment to improve the quality of life in London.

Many stores, including Sainsbury's in Pimlico, where the scheme was launched, already allow passersby to use their loos without buying so much as a packet of chewing gum. The trouble is that most people do not know that such facilities are open to the public – something that Johnson is trying to put right by providing information about where the nearest accessible loo is located.

The mayor believes willing businesses could in turn benefit from the potential customers the scheme will bring through their doors.

Johnson said: "Letting people know where they can use the toilet, and encouraging businesses to open their facilities to the public, will not only make the city a more civilised and pleasant place to live but also to visit."

Ian Clement, one of Johnson's deputy mayors, said the new scheme was a prime opportunity to flag up a private-sector solution to a very private matter.

A keen coffee drinker, Clement had sympathy with those desperate for a nearby loo. "This is something that affects all of us," he said, pointing out that not everyone has the confidence to march into a shop or bar simply to use the toilet without permission.

An Open London page on the mayor's website will list the willing companies. Web links will allow people to check on individual company websites about which outlets have accessible toilets. There will also be a link to Transport for London's toilet map.

Clement sees the initiative as an opportunity to open not just more toilet doors but a wider debate.

"This is also highlighting the issue," he said. "People will start talking about it. We are trying to come up with an increase in provision."

He added that Johnson will also seek to encourage London boroughs to put more provision in place as part of his London Plan, which is due to be published later this year.

Mike Bone, the director of the British Toilet Association, which campaigns for better toilets, welcomed Johnson's scheme to increase access but warned that it risked letting London boroughs off the hook about their own responsibility in providing a clean and accessible service for residents and visitors to their area.

Unlike public conveniences, access to loos on commercial premises will be circumscribed by a shop's opening hours.

Bone cited the community toilet scheme in Richmond upon Thames, now replicated elsewhere, which pays companies £600 a year to open their loos to the public but which now has just five public toilets of its own in the entire borough despite having more than 182,000 residents and attracting 4.5 million visitors a year.

In March, three will be axed with the remaining two under review. "There should be an obligation on local authorities to provide basic toilets," said Bone. "If local authorities want to supplement that with toilets provided by commercial providers, that's good, but it is not a substitute for providing their own public toilets."

The paucity of public service provision for such a basic human need has sparked two public investigations in the past three years: one by a London assembly committee, the other by the Commons select committee for communities and local government. The MPs' report, published last October, noted that the decline in ­provision has seen anxiety about leaving the house rise among vulnerable groups, such as the elderly. Many public conveniences in existence were poorly signposted, the MPs found.

Though government guidance encourages councils to provide good local provision, local authorities have only a discretionary duty to provide public toilets. The select committee's report called for this to be changed to impose a duty on councils to make provisions for public toilets in their area.